The Stanford team of Bendavid and Bhattacharya that was arguing back in March that we were closer to herd immunity than was thought have published in the Journal of the American Medical Association their Los Angeles County study of antibody rates as of April 10-11 from a semi-random study.
May 18, 2020
Seroprevalence of SARS-CoV-2–Specific Antibodies Among Adults in Los Angeles County, California, on April 10-11, 2020
Neeraj Sood; Paul Simon; Peggy Ebner; Daniel Eichner, Jeffrey Reynolds; Eran Bendavid; Jay Bhattacharya
JAMA. Published online May 18, 2020.
… We conducted serologic tests in a community sample to estimate cumulative incidence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, as serologic tests identify both active and past infections. …
Of 1952 individuals invited to participate in antibody testing, 1702 (87.2%) provided consent and 865 (50.9%) were tested. Those not tested could not schedule testing or did not appear. Two test results were inconclusive due to faulty test kits and were removed from the analysis sample. Of 863 adults included, 60% were women, 55% were aged 35 to 54 years old, 58% were white, and 43% had yearly household incomes greater than $100 000. Thirteen percent reported fever with cough, 9% fever with shortness of breath, and 6% loss of smell or taste (Table).
Thirty-five individuals (4.06% [exact binomial CI, 2.84%-5.60%]) tested positive. The fraction that tested positive varied by race/ethnicity, sex, and income (Table). The weighted proportion of participants who tested positive was 4.31% (bootstrap CI, 2.59%-6.24%). After adjusting for test sensitivity and specificity, the unweighted and weighted prevalence of SARS-CoV-2 antibodies was 4.34% (bootstrap CI, 2.76%-6.07%) and 4.65% (bootstrap CI, 2.52%-7.07%), respectively.
So, I would summarize that as being 4% to 5% tested positive
In this community seroprevalence study in Los Angeles County, the prevalence of antibodies to SARS-CoV-2 was 4.65%. The estimate implies that approximately 367 000 adults had SARS-CoV-2 antibodies, which is substantially greater than the 8430 cumulative number of confirmed infections in the county on April 10.3 Therefore, fatality rates based on confirmed cases may be higher than rates based on number of infections.
As of today, 5+ weeks later, the official CV death count for Los Angeles County is 1,840. So 1,840 deaths divided by 367,000 infections would be an Infection Fatality Rate of 0.5% in L.A. County. However, the lag of almost 40 days since testing means that some of the dead as of today were infected after testing. So 0.50% would represent an upward bound on IFR and the apples to apples comparison to other IFR estimates would be somewhat lower. As of 28 days after testing (a more typical lag), the IFR might have been in the 0.35% to 0.40% range.
So L.A. County’s IFR is a little lower than estimates from antibody tests for most other localities I’ve posted, which have typically been between 0.5% and 1.1%.
Hospitals never came close to being overwhelmed in Los Angeles County. And the peak in deaths, in the second half of April, came somewhat later than in other places like NYC, so L.A. doctors had a chance to learn from the struggles of other regions about what not to do. Hopefully, Infection Fatality Rates will fall over time as medical science progresses. Unfortunately, I haven’t seen any studies yet trying to assess whether that is happening or not.
In addition, contact tracing methods to limit the spread of infection will face considerable challenges.
This study has limitations. Selection bias is likely. The estimated prevalence may be biased due to nonresponse or that symptomatic persons may have been more likely to participate. Prevalence estimates could change with new information on the accuracy of test kits used. Also, the study was limited to 1 county. Serologic testing in other locations is warranted to track the progress of the epidemic.